Can I take the Lord's Supper if I've had a bad week spiritually?
It depends on the transaction of the moment, not the quality of the week gone by.
Nobody brings a successful week to the Lord's Table, period. Nobody. We all call into question—and rightly—the effectiveness of our devotions or the quality of our communication with our kids. It's never been perfect. Therefore, we bring to the table our sin.
That's the point of the table. It is a recognition of our sin.
However, what you do in preparation—when you take stock of yourself—is that you confess all known sin. You do Psalm 19: "Cleanse me of hidden faults, and hold back your servant from presumptuous sins."
So you pray specific confession for the sins you know, you pray general confession for the sins you're unaware of, and you receive afresh the cleansing, the application of the blood of Christ (1 John 1:9: "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness").
Now, after you've appropriated afresh the work of Christ and are enjoying that forgiveness, you eat. And you eat worthily, not because you had a good week, but because you have a great Savior and are united with him by faith and are renouncing all those sins.
That's what I encourage our people to do. "Set it right with God now, in these next three minutes." And then as the trays come we celebrate that, we remember the foundation of that forgiveness, by eating.